Research on organic monolayers has been continued over many years, and LB (Langmuir-Blodgett) films, organothiol films, organosilane films and the like are known. As such organic monolayers can achieve surface modifications without affecting the surface morphology (microstructures, planarities, and the like) of substrates or base materials (hereinafter simply called “substrates”) owing to their molecular-height thicknesses, their full-scale exploitation is expected keeping in step with an increasing demand for miniaturization of semiconductors and microprocessing of MEMS (microelectromechanical systems), electronic parts and components, and the like.
Among these organic monolayers, monomolecular organosilicon films which make use of organosilicon compounds each having one or more hydrolyzable groups as film-forming materials can be readily formed as stable monolayers, because these organosilicon compounds can form covalent bonds with hydroxyl-containing materials. Accordingly, many reports have been made on them (see, for example, Patent Documents 1, 2, and 3). Different from the above-described LB films and organothiol films, a monomolecular organosilicon film makes use of hydroxyl groups on a surface of a solid material, said surface being to be treated, as reaction sites with its film-forming compound.
As a precursor for organosilicon monolayer, silicon compounds susceptible to hydrolysis such as alkoxysilanes and chlorosilanes are used. These compounds can directly interacts with surface hydroxyl groups of a solid, leading to dealcoholation or dehydrochlorination. Here, particularly in the case of allkoxysilanes, the dealcoholation is slow, rendering them to be hydrolyzed by water in the system to afford silanols first. Thereafter, the silanols in turn reacts with surface hydroxyl groups of a solid to form chemical bonds. Thus, the direct precursor seems to be silanols. However alkoxysilanes are the practical precursor of choice. The reason of the choice is that silanols may give irreproducible results of monolayer formation and may not be used efficiently as precursor, owing to their general thermal instability compared with alkoxylsilane counterparts, leading to dehydrative self-condensation to result in disiloxanes and siloxane polymers during storage or in the monolayer forming process. For allowing the above-described hydrolysis reaction of the alkoxysilane and the subsequent reaction with the surface hydroxyl groups to proceed efficiently, it is a common practice to add a catalyst such as an acid or base (see, for example, Patent Documents 1 and 2).
However, this catalyst also acts as a condensation catalyst for the silanol so that the resulting silanol is converted into an inert siloxane.
When chlorosilane is used, prior hydrolysis is not absolutely needed because chlorosilane has high reactivity with surface hydroxyl groups. However, this high reactivity leads to poor chemical stability, and needs exceptional attention upon its storage or use, for example, the use of an anhydrous solvent (see, for example, Patent Document 3). Further, highly toxic and corrosive hydrogen chloride is produced upon formation of a film, so that chlorosilane is hardly usable in applications expected as primary applications of monolayer treatment, such as semiconductors and electronic parts or components.
As another problem of the use, as a precursor, of an organosilicon compound that forms an unstable silanol as described above, there is a potential problem in that the resulting film may no longer be a true monolayer at a part or parts or the entire part thereof. Specifically, a siloxane polymer may be formed by self-condensation of the silanol, and may deposit as coarse particles on a surface of a substrate. Further, disorder may occur in a part or parts of the film structure to form a multilayer film.
Therefore, the above-described problems would all be solved successfully if the surface treatment for the formation of a monolayer would be feasible by using a thermally and chemically stable silanol as a precursor. In general, however, the stability and reactivity of a compound are mutually-contradictory properties, thereby involving a problem that the reactivity drops as the stability increases.
Citation List
Patent Document 1: JP-A H8-337654
Patent Document 2: JP-A H10-151421
Patent Document 3: JP-A H4-221630